In European Patent Publication No. 0,248,103, an adapter assembly is described, by means of which a biosensor, e.g., a pressure sensor, can be implanted into a hole bored in the skull cap with precisely defined depth spacing and with exactly coplanar alignment of its measurement diaphragm relative to the dura mater (hard meninx, hereinafter called "Dura"). As described in said EP-A1-0,248,103 the surgeon, using a stepped trephine, bores a stepped hole in the skull bone, said hole having a lower narrower diameter proximate the Dura and a upper wider diameter. Into this stepped hole, a spring leg sleeve is then inserted, the downwardly depending lower spring legs being urged outwardly on insertion of the biosensor into the spring leg sleeve (or in a more advantageous embodiment according to EP-A1-0,248,103, by means of a previously inserted expansion sleeve), the retaining cams provided on the lower tips of the spring legs engaging the lower inner periphery of the skull bore hole to ensure axially and radially correct positioning of the biosensor.
Adapter assemblies used to position biosensors for measuring the internal pressure in the skull, transdural or subdural p0.sub.2 or pC0.sub.2 or for measuring the metabolism in the upper liquor space are as a rule sized to the average thickness of the skull bone of adults, that is to say the axial lengths of the adapter assembly on the one hand and the housing of the biosensor on the other hand are sized to a skull bone thickness of, for example, 3 to 11 millimeters. Increasingly, however, there is also a need for taking measurements using such biosensors, especially pressure sensors, in small children. The thickness of the skull bone in small children is substantially less and the bone is softer than in adults. Thus, the conventional biosensor adapter assemblies can be used for measurements on small children only when very special precautionary measures are observed or not at all.
It is the object of the invention to provide an improved cranial biosensor adapter assembly over that described in EP-A1-0,248,103, whereby a biosensor can be used without problems especially on small children whose bone thickness is less than 3 mm.